1. Field of the Invention
The present invention relates generally to inkjet printing. More specifically, the present invention relates to inkjet printing using an intermediate transfer medium and an ink containing a wetting agent.
2. Description of the Related Art
Early inkjet printers had several distinct advantages over laser printers. They could print in color and had size and cost advantages. However, in order for the next generation of inkjet printers to compete with laser printers in an office environment, it will be necessary to increase the printing speed and print quality. As a goal, a printing speed of 20 pages per minute is desired. For print quality, a print density of 1.4 as measured on an optical densitometer and a resolution of at least 600 dots per inch are desired.
To achieve these goals and successfully compete with laser printers in the business printer market, it has been proposed to print from an inkjet head onto an intermediate transfer surface (e.g. a drum) and then transfer the image onto a final medium (e.g. paper). Prior attempts at this intermediate transfer type printing have resulted in poor transfer efficiency, that is, the image printed onto the intermediate transfer surface did not completely transfer to the final medium. Some of the material deposited on the intermediate transfer surface remained after transfer. Prior attempts at using an intermediate transfer medium with inkjet printing have also resulted in poor optical density.
The present invention teaches a method of printing which uses an inkjet print head to print an ink containing about 0.01 to about 15 wt. % of a wetting agent onto an intermediate transfer surface to form an image on the intermediate transfer surface. The image is transferred from the intermediate transfer surface to a final medium while the ink is partially wet. The wetting agent may be a 1,2 alkyldiol having 4-10 carbon atoms or a diether alcohol having 6-14 carbon atoms. 1,2 hexanediol and hexylcarbitol, respectively, are particularly suitable wetting agents. If 1,2 hexanediol is used as the wetting agent, the ink may contain about 1.0 to about 5.0 wt. % hexanediol. If hexylcarbitol is used as the wetting agent, the ink may contain about 0.1 to about 2.5 wt. % of hexylcarbitol.
The intermediate transfer surface may be coated with a coating solution. In this case, the ink should have a surface energy different from that of the coating solution by about 10 dynes/cm. Preferably the surface energy of the ink should be 2-9 dynes/cm less than the surface energy of the drum coating solution. The coating solution may contain poly vinyl pyrrolidone, and if so, about 0.01 to about 20 wt. % PVP is suitable, about 0.01 to about 15 wt. % PVP is more preferred and about 2 to about 8 wt. % PVP is most preferred. The PVP should have a molecular weight greater than about 400,000, more preferably greater than about 750,000 and most preferably within the range of from about 850,000 to about 1,150,000.